1. Field of the Invention
The present invention relates to a flexible semiconductor device and a production method therefor. More specifically, the invention relates to a semiconductor device including a semiconductor substrate usable in a bent state. The inventive flexible semiconductor device is applicable, for example, to a driver for a liquid crystal display device.
2. Description of the Related Art
There is an increasing trend toward the size reduction and portability facilitation of electronic devices such as mobile phones. As a result of the ultimate facilitation of the portability, a wearable computer or “wearable PC” has recently been disclosed, which is a combination of a garment and a computer mounted thereon. In order to realize the wearable computer, the semiconductor device is required to have flexibility so as to be used in a bent state for system package solution.
One approach to the realization of the flexible semiconductor device is to employ a thin film chip formation method for a solar cell as disclosed in U.S. Pat. No. 4,727,047. FIGS. 9(a) to 9(f) are diagrams for explaining the solar cell thin film chip formation method.
As shown in FIG. 9(a), a substrate 31 is first prepared as an underlying substrate. The substrate 31 is composed of GaAs, for example, and crystalline GaAs is caused to grow on a surface of the substrate 31 as will be described later. Then, a photolithography process is performed by resist application, exposure and development, and a heat treatment process is performed at 400° C. for about one minute. Through these processes, a mask layer 33 for the crystal growth is selectively formed on the substrate 31 as shown in FIG. 9(b), whereby recesses 35 are formed to expose surface portions of the substrate 31.
In turn, crystalline GaAs is caused to grow on the resulting substrate in an AsCl3—Ga—H2 vapor atmosphere as shown in FIG. 9(c). The crystalline GaAs grows first on the surface portions of the substrate 31 in the recesses 35, then on side faces of the mask layer 33, and horizontally along an upper surface of the mask 33. Thus, grown GaAs layers 36a are formed in the recesses 35, and a crystalline GaAs layer 36 is formed on the grown GaAs layers 36a and the mask layer 33 as shown in FIG. 9(d). Thereafter, an element is formed in the crystalline GaAs layer 36.
Then, the crystalline GaAs layer 36 is separated from the substrate 31 by cleaving the mask layer 33 or by etching away the mask layer 33 with hydrofluoric acid (FIG. 9(e)). Thus, a flexible semiconductor chip 36b is produced as shown in FIG. 9(f).
Further, one approach to the realization of a flexible liquid crystal display is to employ a thin film Si chip formation method for an active matrix liquid crystal display (AMLCD) as disclosed in U.S. Pat. No. 5,702,963.
In this method, the production of the flexible semiconductor device is achieved by employing a silicon-on-insulator (SOI) substrate and a lift-off method. More specifically, the SOI substrate is prepared by forming in this order an Si substrate surface layer, an Si buffer layer for protecting the Si substrate surface layer during lift-off, an underlying oxide layer, a release layer (e.g., an oxide/nitride layer to be separated) having a lower etching rate than the underlying oxide layer, and an upper Si layer in which an element is formed.
Thereafter, a TFT transistor element is formed in the upper Si layer by a known method. Then, the underlying oxide layer present between the release layer and the Si buffer layer is partly removed, and support posts for temporarily supporting the layers present above the release layer are formed in regions devoid of the underlying oxide layer. In this state, the underlying oxide layer is etched away for formation of a cavity. In turn, the upper Si layer formed with the element is sealed with a resin. Finally, the resulting element/substrate is separated by cleaving the support posts. Thus, a flexible chip is produced.
However, the underlying substrate 31 employed in the first prior art (U.S. Pat. No. 4,727,047) is reusable after the process. The surface of the underlying substrate influences the initial stage of the growth of the GaAs layers 36a. Since the grown layers 36a are formed directly on the underlying substrate, the growth of the GaAs layers 36a is likely to be influenced by the morphology and contamination of the surface of the underlying substrate. Therefore, uniform growth of the layers is difficult, so that uneven films may be formed. The formation of the uneven films results in local concentration of stresses in the grown GaAs layers 36a and the crystalline GaAs layer 36. Therefore, the grown GaAs layers 36a and the crystalline GaAs layer 36 are poorer in flexibility and susceptible to cracking.
In the second prior art (U.S. Pat. No. 5,702,963), the upper Si layer (element formation layer) is formed directly on the release layer. In this case, there is a possibility that the upper Si layer is separated during the formation of the element due to poor adhesion between the upper Si layer and the release layer (the adhesion depends on the morphology of the underlying release layer). Further, the semiconductor layer has a relatively uneven thickness and, hence, a poorer flexibility.